How to synchronize the brightness of a grille-mounted neon tube/LED bar to a car's tach/speedo?
I have a 1997 BMW 328i, and I've been thinking about installing either a neon tube or a bar of LED's in the grille of the car to use at night (most likely not street legal, I know). I want the brightness of the tube/bar to be controlled by one of two things: The speedometer or the tachometer.
I.e., if the speedometer is at 0 MPH, the light is off, and if the speedo is at 75 MPH, the light is fully on.
Same thing with the tach, light off at 0 RPM, light fully on at redline.
Unfortunately, I haven't the slightest idea how to use the car's computer or the gauges themselves to set up a control module for the light. If any of you has a good amount of knowledge about automotive gauges, I would appreciate your help!